add restart application
rename to ReplayAppFromLatestTimestamp
rename to ReplayFromTimestampWindowTrailingEdge
Merge pull request #94 from huafengw/master

add restart application
remove tests for this branch
add TimeExtractor and change api
dedicate a thread to fetch and consolidate all messages from a broker
fix message iterator bugs and change configs
fix message iterator and checkpoint bugs
Merge remote branch 'upstream/master' into at_least_once
Merge pull request #96 from manuzhang/at_least_once

refactor checkpoint interface and fix bugs
update tests against recent changes